Recent advances in microwave imaging have enabled commercial development of microwave imaging systems that are capable of generating two-dimensional and even three-dimensional microwave images of objects and other items of interest (e.g., human subjects). At present, there are several microwave imaging techniques available. For example, one technique uses an array of microwave detectors (hereinafter referred to as “antenna elements”) to capture either passive microwave radiation emitted by a target associated with the person or other object or reflected microwave radiation reflected from the target in response to active microwave illumination of the target. A two-dimensional or three-dimensional image of the person or other object is constructed by scanning the array of antenna elements with respect to the target's position and/or adjusting the frequency (or wavelength) of the microwave radiation being transmitted or detected.
Microwave imaging systems typically include transmit, receive and/or reflect antenna arrays for transmitting, receiving and/or reflecting microwave radiation to/from the object. Such antenna arrays can be constructed using traditional analog phased arrays or binary reflector arrays. In either case, the antenna array typically directs a beam of microwave radiation towards a point in 3D space corresponding to a voxel in an image of the object, hereinafter referred to as a target. This is accomplished by programming each of the antenna elements in the array with a respective phase shift. Examples of programmable antenna arrays are described in U.S. Pat. No. 7,224,314, entitled “A Device for Reflecting Electromagnetic Radiation,” and Ser. No. 10/997,583, entitled “Broadband Binary Phased Antenna.”
When using reflector antenna arrays, a typical microwave imaging system includes a microwave source, a microwave receiver, which may be co-located with the microwave source, and one or more reflector antenna arrays. Microwave radiation transmitted from the source is received at the reflector antenna array and reflected towards a target by programming each of the reflecting antenna elements in the array with a respective phase shift. Likewise, reflected microwave radiation reflected from the target and received by the array is reflected towards the microwave receiver by programming each of the individual reflecting antenna elements with a respective phase shift. The microwave receiver combines the received microwave radiation reflected from each antenna element in the array to produce a value of the effective intensity of the reflected microwave radiation at the target, which represents the value of a pixel or voxel corresponding to the target on the object.
However, some of the microwave radiation from the source is reflected off of the array and directly transmitted towards the microwave receiver without reflecting off the target. In addition, some of the microwave radiation from the source is scattered off of various undesired points in 3D space (e.g., other targets on the object being imaged or other objects) towards the array, and reflected back to the microwave receiver. Such stray microwave radiation contributes to the background noise (often referred to as “clutter”), and reduces the signal-to-noise ratio (SNR) of the microwave imaging system. What is needed is a mechanism for minimizing the background noise in a microwave image captured using a programmable reflector array.